Photonic, optical and micro-mechanical devices are typically packaged such that the active elements (i.e., the emitters, receivers, micro-mirrors, etc.) are disposed within a sealed chamber to protect them from handling and other environmental hazards. In many cases, it is preferred that the chamber be hermetically sealed to prevent the influx, egress or exchange of gasses between the chamber and the environment. Of course, a window must be provided to allow light or other electromagnetic energy of the desired wavelength to enter and/or leave the package. In some cases, the window will be visibly transparent, e.g. if visible light is involved, but in other cases the window may be visibly opaque while still being “optically” transparent to electromagnetic energy of the desired wavelengths. In many cases, the window is given certain optical properties to enhance the performance of the device. For example, a glass window may be ground and polished to achieve certain flatness specifications in order to avoid distorting the light passing therethrough. In other cases, anti-reflective or anti-refractive coatings may be applied to the window to improve light transmission therethrough.
Hermetically sealed micro-device packages with windows have heretofore been produced using cover assemblies with metal frames and glass window panes. To achieve the required hermetic seal, the glass window pane has heretofore been fused to its metallic frame by heating it in a furnace at a temperature exceeding the glass transition temperature, TG (typically at or above 900° C.). However, because the fusing temperature is above TG, the original surface finish of the glass pane is typically ruined, making it necessary to finish or re-finish (e.g., grinding and polishing) both surfaces of the window pane after fusing in order to obtain the necessary optical characteristics. This polishing of the window panes requires additional process steps during manufacture of the cover assemblies, which steps tend to be relatively time and labor intensive, thus adding significantly to the cost of the cover assembly, and hence to the cost of the overall package. In addition, the need to polish both sides of the glass after fusing requires the glass to project both above and below the attached frame. This restricts the design options for the cover assembly with respect to glass thickness, dimensions, etc., which can also result in increased material costs.
Once a cover assembly with a hermetically sealed window is prepared, it is typically seam welded to the device base (i.e., substrate) in order to produce the finished hermetically sealed package. Seam welding uses a precisely applied AC current to produce localized temperatures of about 1,100° C. at the frame/base junction, thereby welding the metallic cover assembly to the package base and forming a hermetic seal. To prevent distortion of the glass windowpane or package, the metal frame of the cover assembly should be fabricated from Kovar alloy or another alloy having a CTE (i.e., coefficient of thermal expansion) which is similar to that of the transparent window material and to the CTE of the package base.
While the methods described above have heretofore produced useable window assemblies for hermetically sealed micro-device packages, the relatively high cost of these window assemblies is a significant obstacle to their widespread application. A need therefore exists, for package and component designs and assembly methods which reduce the labor costs associated with producing each package.
A need still further exists for package and component designs and assembly methods which will minimize the manufacturing cycle time required to produce a completed package.
A need still further exists for package and component designs and assembly methods which reduce the number of process steps required for the production of each package. It will be appreciated that reducing the number of process steps will reduce the overhead/floor space required in the production facility, the amount of capital equipment necessary for manufacturing, and handling costs associated with transferring the work pieces between various steps in the process. A reduction in the cost of labor may also result. Such reductions would, of course, further reduce the cost of producing these hermetic packages.
A need still further exists for package and component designs and assembly methods which will reduce the overall materials costs associated with each package, either by reducing the initial material cost, by reducing the amount of wastage or loss during production, or both.